A very highly efficient heat pump or refrigeration unit can be constructed by connecting a free piston Stirling engine to a compressor and driving the unit with thermal input energy, such as from gas fuel. Such an apparatus includes multiple reciprocating masses, interconnected together and to ground by means of effective springs and operates typically in resonance. An inherent operating characteristic of the simple free piston Stirling engine is that its output stroke amplitude increases as the power delivered by the engine increases when other operating parameters don't vary. Therefore, in the absence of compensating structure, as the load upon the engine is reduced, its stroke will ordinarily increase and can increase sufficiently that damage to the mechanical components can result. This problem is particularly difficult when a free piston Stirling engine is used to drive the compressor of a refrigeration unit. The power demand of such a load will vary as the result of variations in the ambient temperature or other conditions effecting heat transfer into the cooled chamber or from the heat exchanger and condenser or as a result of normal cycling of the refrigeration unit as it maintains the cooled chamber between upper and lower temperature limits. Because of the relatively slow reaction time of the heated masses within the free piston Stirling engine, the thermal response time is relatively slow so that it becomes impractical to reduce the heat energy input as a means for reducing the power output of the free piston Stirling engine.
Therefore, it is desirable to provide a compensating apparatus which will quickly reduce the engine output power when a reduced load is encountered and quickly increase the output power when an increased power is demanded by the load. Ideally the compensating apparatus would continuously match the engine output power to the load power demand throughout a wide operating range and maintain engine reciprocation amplitude relatively constant so that critical clearances can be maintained.